Method and apparatus for providing biasing criteria for binary decisions for use in wireless communications to enhance protection

ABSTRACT

A criterion for biasing a binary decision requiring an unequal protection which utilizes a measured signal to interference ratio (SIR). The SIR may be derived from a determination of channel estimation. The SIR is compared against a threshold, the threshold being selected to bias the decision toward a NACK as opposed to an ACK determination. The technique is advantageous for providing biased binary decisions for high speed downlink packets (HSDP) but may be utilized for both uplink and downlink applications.

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority from U.S. non-provisionalapplication No. 60/357,947 and filed Feb. 19, 2002, which isincorporated by reference as if fully set forth.

FIELD OF INVENTION

[0002] The present invention relates to wireless communications. Moreparticularly the present invention relates to providing biased binarydecisions for high speed downlink packets (HSDP).

BACKGROUND

[0003] In HSDP HSDPA, as well as many other communication systems, thereare applications where a binary decision is required but wherein thepenalty of error is unequal as between the two decisions.

[0004] For example, HSDPA uses an ACK/NACK signal where the userequipment (UE) indicates whether or not a transmitted block of data hasbeen successfully decoded. It has been recognized that it is moreinjurious to system performance for a NACK message to be incorrectlyinterpreted as an ACK than for an ACK message to be incorrectly decodedas a NACK. In the latter case, the error would result in a transmissionblock being unnecessarily retransmitted; which amounts to only a smallloss in efficiency. In the former case, the transmitting side wouldassume success for the previously transmitted block, and would notresend it. This is a catastrophic failure, causing serious systemdisruption.

[0005] Several obvious techniques have been recognized to bias theanswer in favor of identifying the condition with the NACK. For example,in normal coherent demodulation of binary phase shift keying (BPSK), theoutput signal is often normalized, e.g., an ideal signal representing 1is +1.0, while an ideal signal representing 0 would be −1.0 and, in atypical channel, which has impairments, interference and noise, thenormalized output may take on any value therebetween.

[0006] In an unbiased decision, if an output z>0 then the processdeclares 1, and, if the output z<0 then the process declares theoutput=0. In a biased decision, if the output z>X then declare 1,otherwise declare 0. X is identified as the threshold value and isselected based on the analysis. If it is desired to favor the output 1,then X will be negative; e.g., a small negative number; e.g., minus 0.1,etc.

[0007] Employing the above process, normalizing the output and derivingthe optimum threshold can be delicate and complex to implement and issubject to degradations due to tolerances in the implementation.

SUMMARY OF THE INVENTION

[0008] In accordance with the present invention a criterion is utilizedwhich makes the decision much simpler and more reliable in application,which criterion is significantly more reliable than the simple Xthreshold.

[0009] In accordance with this criterion, for time division duplex(TDD), the criterion is based on the measured signal to interferenceratio (SIR), which is derived from the channel estimation process. Itcan be shown that virtually all instances of error occur when the SIRvalue is low.

[0010] Correlation between failures, or near failures of the radiomodulation (RM) decoder and errors in the other bits of the transmissionis high; therefore, the criterion of the present invention is highlyreliable and can be easily implemented.

BRIEF DESCRIPTION OF THE FIGURES

[0011]FIG. 1 is a diagrammatic representation of apparatus forpracticing the novel methods of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0012]FIG. 1 shows a routine utilized to practice the technique of thepresent invention. In the example shown, which is a downlink example, adevice such as a base station (BS10), based on a communication fromUE12, provided at step S1, utilizes this communication to obtain channelestimation, at step S2.

[0013] The BS10 derives a signal to interference ratio (SIR), from thechannel estimation, at step S3 using well known techniques. At step S4BS10 compares the SIR obtained at step S3 with a given threshold.

[0014] The threshold may be determined by employing any one of a varietyof different techniques. One technique which may be used is todetermine, through a combination of analysis, aided by simulation, andactual testing, with a test set-up. Test signals plus noise are input tothe test set-up and output signals are examined. The output signals aremeasured and the correctness of the binary decision is also determined.This test is repeated over an entire range of useful signals, includingvery low signals. For each amplitude level, the number of instances ofcorrect decisions and incorrect decisions are stored. Generally, strongsignals are associated with very high frequency of correct decisions andweak signals are associated with high instances of incorrect decisions.The threshold is preferably greater than the level of noise taken alone.

[0015] Since the objective is to avoid one particular type of error;e.g. probability of declaring an ACK, given that a NACK was sent, thethreshold is selected i.e., biased, so that, when signals are above thethreshold, there is an acceptably low frequency of errors and whensignals fall below this threshold there is an unacceptable frequency ofoccurrence of errors. Since the objective is to avoid false declarationsof ACK, any decision that occurs with a signal below the threshold isdeclared as a NACK. More generally, when the signal is below thethreshold the binary decision is thus biased to be the choice thatcauses the least catastrophic result, even if in error. This techniquemay be performed either off-line or periodically by the BS10 (or UE12),as shown by step S11. Also, if desired other techniques may be used toobtain a threshold value.

[0016] If the SIR is less than the threshold, BS10, at step S5,transmits a NACK signal to UE12. UE12, at step S6 receives and detectsthe NACK signal and, at step S7 retransmits the temporarily storedcommunication received by BS10 at step S8.

[0017] Returning step S4, assuming that the SIR is greater than thethreshold value, the routine branches to Step S9 to transmit an ACKsignal. The UE12 at step S10 receives the ACK signal and clears thecommunication temporarily stored at step S1.

[0018] It should be understood that the routine shown in FIG. 1 isequally applicable to uplink wherein the functions performed by the BS10and UE12 are reversed, and it is the BS10 that transmits and temporarilystores a communication at step S1. The UE12 obtains channel estimationat step S2, derives the SIR, at step S3, compares the SIR derived with agiven threshold, at step S4, and transmits a NACK to the BS at step S5.BS10, receiving the NACK and retransmits the temporarily storedcommunication at steps S6 and S7 respectively, the retransmittedcommunication being received by the UE12 at step S8. In the event thatthe SIR is greater than the threshold setting, the UE transmits an ACKcondition at S9, which is received by the BS10 at step S10 whereby theACK signal causes the BS10 to clear the temporarily storedcommunication.

[0019] It should be understood that the technique of the presentinvention may also be used simultaneously in both uplink and downlinkcommunications between a UE and a BS.

What is claimed is:
 1. A method for biasing criteria for binarydecisions requiring unequal protection in a wireless communicationsystem, comprising: receiving a communication in a given channel;obtaining a signal to interference ratio (SIR) of the communicationchannel; and comparing the SIR against a given threshold, said thresholdbeing selected to bias the criteria toward a transmission of a (NACK)condition to indicate that a received communication should beretransmitted.
 2. The method of claim 1 wherein said SIR is obtained byinitially obtaining a channel estimation and deriving the SIR from thechannel estimation.
 3. The method of claim 1 wherein the step ofcomparing comprises: determining the threshold by combining, with noise,each of a plurality of test signals having signal strengths extendingover a given range; determining correctness of each combined signal;determining instances of correct signals and incorrect signals at eachsignal strength; and selecting a threshold which provides an acceptablenumber of false declarations of an ACK condition.
 4. The method of claim3 wherein the comparing step includes using signal amplitude as ameasure of signal strength.
 5. The method of claim 3 wherein thethreshold is preferably greater than a level of the noise taken alone.6. The method of claim 1 wherein the step of comparing comprises:setting the threshold to a level that is low enough so that anacceptable low frequency of errors occurs for SIRs above the thresholdand high enough to prevent an unacceptable high number of errors forSIRs below the threshold.
 7. The method of claim 1 wherein the channelestimation is based on a communication from a sender.
 8. The method ofclaim 3 wherein the sender temporarily stores the communication sent tothe receiver.
 9. The method of claim 8 wherein the sender, upon receiptof a NACK signal retransmits the temporarily stored communication. 10.The method of claim 1 wherein the receiver transmits an ACK conditionacknowledging receipt of an error free communication.
 11. The method ofclaim 8 wherein said sender, upon receipt of an ACK signal clears thetemporarily stored communication.
 12. Apparatus for biasing criteria forbinary decisions requiring unequal protection in a wirelesscommunication system, comprising: means for receiving a communication ina given channel; means for obtaining a signal to interference ratio(SIR) of the communication channel; and means for comparing the SIRagainst a given threshold, said threshold being selected to bias thecriteria toward a transmission of a (NACK) condition to indicate that areceived communication should be retransmitted.
 13. The apparatus ofclaim 12 wherein said SIR is obtained by means for initially obtaining achannel estimation, said means for obtaining deriving the SIR from thechannel estimation.
 14. The apparatus of claim 12 wherein the means forcomparing comprises: means for determining the threshold by combining,with noise, each of a plurality of test signals having signal strengthsextending over a given range; means for determining correctness of eachcombined signal; means for determining instances of correct signals andincorrect signals at each signal strength; and means for selecting athreshold which provides an acceptable number of false declarations ofan ACK condition.
 15. The apparatus of claim 14 wherein the means forcomparing further comprises means using signal amplitude as a measure ofsignal strength.
 16. The apparatus of claim 14 wherein the threshold ispreferably greater than a signal strength of a noise signal taken alone.17. The apparatus of claim 12 wherein means for initially obtaining thechannel estimation is based on a communication means of a sender. 18.The apparatus of claim 13 wherein the sender's communication meansincludes means for temporarily storing the communication sent to thereceiver.
 19. The apparatus of claim 18 wherein the sender is providedwith means for receiving a NACK signal and means to retransmit atemporarily stored communication responsive to the means for receiving.20. The apparatus of claim 12 further comprising means for transmittingan ACK condition acknowledging receipt of an error free communicationwhen the SIR is greater than the threshold.
 21. The apparatus of claim12 wherein means are provided for receiving an ACK condition and meansfor clearing a temporarily stored communication responsive to said meansfor receiving an ACK condition.
 22. The apparatus of claim 12 whereinthe means for comparing comprises: means for setting the threshold to alevel that is low enough so that an acceptable low frequency of errorsoccurs for SIRs above the threshold and high enough to prevent anunacceptable high number of errors for SIRs below the threshold.